The present invention relates to a method for the determination of the time position of a periodically recurring signal.
Within, for example, the technical fields of communication and radar there often arises a need to determine the time position of a periodically recurring signal with a noise or interference contribution. The periodic signal is, for example, a radar scan of 360.degree., see FIG. 1, where in a certain direction .OMEGA. there is a target echo (10) and in other directions there are interference echoes (11). In such cases it is highly desirable to be able to determine the direction of the target echo (10) in spite of the influence of interferences (11).
In a communication signal the item searched for can be a periodically recurring synchronization word.
Mobile telephony is an area among many where it is possible to find communication signals of different sorts. To be able to use different transfer media in the complete transfer chain within mobile telephony to their full capacity, time multiplexed signals are frequently used. This is because the transfer capacity of a transfer medium often exceeds the amount of data that a single mobile telephone can generate or receive. Time multiplexing requires that a transmitter and a receiver are synchronized so that a mobile telephone does not transmit into or receive from someone else's time slot as this would lead to unsurmountable problems. To synchronize a transmitter and a receiver, synchronization words are frequently used that reappear with a certain periodicity that depends upon the specific application. Synchronization words can consist of anything from single bits or pulses via symbols to larger synchronization words with a plurality of symbols.
During normal conditions a transmitter and a receiver are fairly synchronized and only a simple tracking function is needed, for example in the form of a tracking window that finds the exact position of the synchronization words to thus ensure that the complete transfer chain is synchronized. During certain conditions it is possible to lose the synchronization to such a degree that a tracking window no longer manages to find the position of the synchronization words. In such a situation reflections of the signal may appear, which worsens the possibilities even further of being able to find the correct synchronization word. The synchronization in the transfer chain gets lost and a transfer of information becomes completely impossible in such cases. Such situations can happen, for example, when a change of base stations is effectuated, i.e. a transfer from a base station where synchronization exists to another base station.
Normally, it is possible to find the time position of the signal by creating a window around the area where the signal is expected. However, in a radar it is not known where the signal can be expected and in a communication signal it is difficult when the synchronizing words are lost, i.e. when the synchronization is lost.
One way to separate targets from interferences in a radar is described in the U.S. Pat. No. 4,649,394. This patent describes a method that determines different, suitable thresholds by creating different average values of signals in the region of the target to thus be able to separate targets from interferences. This method does not describe how to determine the time position of arbitrary cyclic functions, nor does the method seem to be suitable in any other area than radar.